The present invention relates to electrodes for use in secondary electrochemical cells. More particularly, it concerns a negative electrode composition and cell including a lithium-aluminum alloy and an additive which exhibits an increased capacity and lithium-aluminum alloy utilization over that of a lithium-aluminum electrode without the additive.
A substantial amount of work has been done in the development of high-temperature, secondary electrochemical cells. Positive electrodes for these cells have included chalcogens such as sulfur, oxygen, selenium or tellurium as well as their transition metal chalcogenides. Positive electrode materials such as the sulfides of iron, cobalt, nickel and copper are of current interest.
In high-temperature cells, current flow between electrodes often is transmitted by molten electrolytic salt. Particularly useful salts include compositions of the alkali metal halides and/or the alkaline earth metal halides ordinarily incorporating a salt of the negative electrode reactant metal, e.g. lithium. In cells operating at moderate temperatures, aqueous and organic base electrolytes are permissible and these also can include cations of the negative electrode metal.
Alkali metals such as lithium, sodium, potassium or alkaline earth metals including calcium, magnesium, etc. and alloys of these materials are contemplated as negative electrode reactants. Alloys of these materials such as lithium-aluminum, lithium-silicon, lithium-magnesium, calcium-magnesium, calcium-aluminum, calcium-silicon and magnesium-aluminum have been investigated to maintain the negative electrode in solid form and thereby improve retention of the active material at high cell operating temperatures.
One of the disadvantages of previous lithium alloys, e.g. lithium-aluminum electrodes, has been the decline in cell capacity during prolonged operation. The reduced capacity has been accepted in order to obtain the enhanced electrode and cell stability afforded by solid lithium alloys. In lithium-aluminum negative electrodes, postoperative examinations of long-lived cells have revealed high lithium concentrations at the negative electrode face and agglomeration of Li-Al particles in the porous electrode.
The present inventor has found that the addition of a small amount of graphitized carbon or intermetallic Al.sub.5 Fe.sub.2 or mixtures thereof to a lithium-aluminum electrode has the unexpected properties of significantly increasing the cell capacity and stability over multiple discharges and of significantly increasing the lithium-aluminum utilization.